1. Field of the Invention
The invention concerns electromagnetic lumped constant delay lines.
Electromagnetic delay lines are quadripoles; the purpose of which is to delay the signals applied to their input by a constant period. They are set by the series connection of several elementary networks comprising a self-inductance constant and a capacitive constant. For electromagnetic delay lines with localized constants, these constants are those of discrete components (inductances and capacitors) connected to one another in different ways (K constant network, derived m network, etc.). The choice of the type of network used is laid down by the performance characteristics to be obtained and by the ease and cost of manufacture.
2. Description of the Prior Art
The ideal characteristics expected of a delay line comprise a constant weakening, as a function of the frequency of the injected signal, which is also a minimal weakening, a constant propagation time as a function of frequency and a constant phase shift, again as a function of frequency.
The so-called constant K network cannot be used except in a range of frequencies far below the cut-off frequency of the quadripole forming the network, namely practically down to a frequency equal to 0.15 times the cut-off frequency if it is sought to preserve a constant delay to within 1%.
On the basis of the constant K network, it is possible to define the two sorts of derived networks called derived m networks: series derived networks and parallel derived networks.
A derived m network has the advantage, as compared with a constant K network, of having a low variation in the delay as a function of the frequency, for values of m close to 1.275. For this value, the delay remains constant, to within 1%, up to a frequency equal to 0.55 times the cut-off frequency. This enables the derived m network to be used in a range of frequency which is 3.6 times wider than for the constant K network. The characteristic impedance shows, for the values of m close to 0.6, a relative variation of less than 5% in the frequency range extending from 0 to 0.85 times the cut-off frequency.
The m network thus enables two important results to be obtained: the smoothing of the delay curve (for m=1.275) and the smoothing of the characteristic impedance curve (for m=0.6). To have a delay line possessing both these characteristics, derived m networks with m=1.275 will be cascade-connected with, at the ends of the cascade, two m half networks with m=0.6.
There are several known ways of making delay lines of this type. These delay lines are differentiated by the method of coiling the chokes.
The chokes can be wound in the form of a solenoid on a cylindrical rod. However, in certain cases (the case of a low or high inductance value), this may lead to unacceptable coiling dimensions or to an excessively high resistive value of the inductance. One way of coping with this problem is to do the coiling on a magnetic support with grooves designed to take the windings. The goal sought is generally achieved, but the coiling is still difficult and expensive. It is also possible to use separate magnetic supports (rings, bars or pulleys) sufficiently spaced out between one another to prevent mutual inductance and each coiled with an intermediate connector. This embodiment has the advantage of giving the designer total liberty as regards the geometrical arrangement of the coils. However, the fabrication of the inductances is very delicate and costly. Besides, the series arrangement of the mutually coupled coils raises problems related to the amount of space occupied by the delay line thus made when their number is great. The component thus obtained is very long and unnecessarily too narrow. It becomes impossible to implant it in a printed card. It is then preferred to arrange the coils on two parallel magnetic rods in order to reduce the longitudinal space factor.
The electromagnetic delay line may then be fabricated in a DIL (dual in-line) package. The total space factor is not excessive, and the pin unit used (DIL 16 for example) is universal. However, the passage from one magnetic bar to another magnetic bar causes a break in coupling. It follows therefrom that the electromagnetic delay thus formed is actually a set of two sections of delay lines, cascaded without any special precautions. This series connection of two electromagnetic delay lines which, are efficient when used alone, include the drawbacks of a break in impedance at the link between two sections, causing stray reflections and distortions, an undulation of the delay curve and inequality in the spans of the intermediate delays.
The previously mentioned defects concerning the series connection of electromagnetic delay lines, comprising several sections, are encountered whenever it is attempted to cascade several, individually efficient, electromagnetic delay lines. While these defects can be tolerated in many instances of standard use, they are strictly unacceptable for specific applications where evenness of the amplitude/frequency, delay/frequency and impedance/frequency responses is indispensable (for example in medical echography).
In order to overcome these drawbacks, the invention proposes an electromagnetic lumped constant delay line with using networks of the derived m type and made in the form of several cascaded sections, without coupling breaks between networks. The delay line according to the invention can be prepared on a printed circuit or on any other substrate, without any coupling break between the networks within one and the same delay line, nor any coupling break with the networks of the input and output ends of the line.